1. Field of the Invention
The invention relates broadly to molding technology. More specifically, this invention relates to ophthalmic lens molding processes. In a preferred embodiment, the invention relates to contact lens molding processes.
2. Description of the Related Art
The formation of articles by dispensing a polymerizable and/or crosslinkable liquid material into a mold has been used in a wide variety of technological areas. Of particular interest is the molding of medical devices, such as ophthalmic lenses. One type of ophthalmic lens which is widely used for vision correction is the contact lens.
Contact lenses are manufactured by a number of processes. One traditional approach of lens manufacture is to form a lens blank by polymerization of liquid monomers in a lens blank mold, then mechanically lathe the lens blank into a finished contact lens. Typically these lathed lenses are subjected to a subsequent polishing step to remove imperfections generated during the lathing process. More recently, double-sided molding (DSM) processes have been developed. These processes typically involve dispensing of a liquid monomer into a female mold half, mating a male mold half to the female, and then applying ultraviolet radiation to polymerize the monomers. The polymerized lens removed from the molds in a DSM process does not usually require surface polishing, but subsequent extraction of unreacted monomer or solvent is commonly required.
U.S. Pat. No. 5,508,317, issued to Beat Muller on Apr. 16, 1996, discloses remarkable improvements in the chemistry of the polymerizable material for molding ophthalmic lenses by providing a method of molding contact lenses without the need to remove unreacted monomer or solvent. This patent disclosed a water soluble prepolymer composition which can be dispensed into lens molds and crosslinked to form a finished optical-quality contact lens in a matter of seconds, without the necessity for subsequent extraction steps. The improved chemistry taught by this patent enabled substantial cost reductions in contact lens manufacture, thereby reducing costs sufficiently to make daily disposable contact lenses a reality for consumers.
European Patent Application No. 637,490, published on Feb. 8, 1995, discloses some outstanding methods of molding the prepolymer materials of U.S. Pat. No. 5,508,317. One embodiment of the invention involves introducing crosslinkable material into a two-part mold, wherein the mold halves are held a small distance from one another so that a thin annular gap is formed between them. The gap is in fluid communication with the mold cavity, so excess prepolymer may escape through the gap. Crosslinking of the prepolymer occurs by application of radiation, e.g., UV light, with the impingement of the radiation being restricted to the mold cavity by masking, i.e. blocking light impingement, in the areas outside the mold cavity. The molding teachings of U.S. application Ser. No. 08/274,942, Hagmann, et al., is incorporated herein by reference.
However, improvements in the efficiency of molding processes are always desirable. Thus, there is a need for molding production process improvements which reduce product cycle time, increase finished product quality, improve product consistency, and reduce the consumption of processing materials. In addition, reductions in environment impact are always desirable, e.g. by reducing the number of molds which must be recycled and/or trashed.